This invention relates generally to fiber optic chemical sensors and more particularly to the pH-sensing hydrogel (dye/gel polymer) used in such sensors.
pH-sensors are based on the principle that certain materials' optical properties change with pH. For example, D. Lubbers et al., "Nanoencapsulated Fluorescence Indicator Molecules Measuring pH and pO.sub.2 Down to Submicroscopical Regions on the Basis of the Optode-Principle", Z. Naturforsch., 32c, 133-134, 1977, used a fluorescent material (.beta.-methyl umbelliferone) encapsulated in polymer beads having porous outer membranes to measure pH. The fluorescent intensity of the material is proportional to pH. Peterson et al., "Fiber Optic Probe for Physiological Use", Anal. Chem., 52, 864-869, 1980, used a colorimetric pH-indicating dye (eg. phenol red) bound to polyacrylamide hydrogel by copolymerization of the dye with the acrylamide monomer in the preparation of their fiber optic pH-sensing probes. The dye changes color with changes in pH, and the color change corresponds to a change in the amount of light of a specified wavelength absorbed by the dye (eg. green light in the case of phenol red). The Peterson et al. fiber optic probe (also described in U.S. Pat. No. 4,200,110) as shown in FIG. 1 consists of an ion permeable membrane 11 which encloses the distal ends of a pair of optical fibers 12, 13. Retained within the hollow membrane 11 and distal to the distal ends of the optical fibers is the pH-indicating sensor (dye/gel polymer) 14. Fiber 12 provides light to sensor 14 and fiber 13 transmits the absorbable light from sensor 14. Thus, the pH of a solution placed in contact with a column of sensing gel, through the ion permeable membrane, can be determined by the intensity of the absorbable light transmitted through the optical fibers.
pH sensors and pH-based sensors which are used for applications such as continuous patient monitoring are typically manufactured, calibrated, and then sold for use. Once used, the sensors are thrown out. pH sensitive dye/gels change color or fluorescence as a result of changes in the pH of the solution. For analytical purposes it is critical that the color or fluorescent intensity of the dye/gel be constant for a given pH. Sensors made with the conventional dye/gel polymer show a substantial drift in measured pH with time, resulting from a continual decay in the absorbence of the dye/gel. This drift also corresponds to an increase in the protonation constant, K, of the gel, and severely limits the accuracy of the sensors for pH measurements over extended periods of time (i.e., more than a few hours), or necessitates frequent recalibration of the probes. Furthermore, the initial optimum operating range of these sensors is variable and also drifts with time because of changes in K. All of these factors contribute to a serious lack of reliability in pH-sensors which impedes their use for continuous patient monitoring.
Therefore, it is an object of the present invention to provide a dye/gel polymer suitable for use as a pH-sensor having minimal drift.
It is another object of the present invention to provide a dye/gel polymer, having a consistently repeatable K value, suitable for use as a pH-sensor.
It is yet another object of the present invention to provide a chemically stable pH-sensing probe, having minimal drift, suitable to be implanted in tissue for physiological studies.
It is also an object of the present invention to provide an optically stable pH-sensing probe, i.e. one in which the color or fluorescence is stable for a given pH.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.